1. Field of the Invention
The present invention is generally related to the repair of pressure vessels and more particularly to plugging penetrations in pressure vessels.
2. General Background
The pressurizer in a nuclear reactor coolant system establishes and maintains the reactor coolant system pressure within the prescribed limits of the system. It provides a steam surge chamber and a water reserve to accommodate reactor coolant density changes during operation. A typical pressurizer is a vertical, cylindrical vessel with replaceable electric heaters in its lower section. The electric heaters are positioned below the normal water line and are actuated to restore normal operating pressure when the pressure in the reactor coolant system has decreased.
The electric heaters are comprised of a plurality of heating elements that extend through nozzles or sleeves through the wall of the pressurizer. The sleeves extend outward from the pressurizer to provide exterior support to the heating elements. The sleeves may be formed from an inconel sleeve roll expanded and welded into the pressurizer. Due to the severe operating environment, cracks have developed in the rolled/welded region of some sleeves. This results in leakage of primary system fluid which is unacceptable in a nuclear steam supply system. Although the ideal situation is to make a permanent repair by replacing the damaged sleeve, this option is not always readily available due to the cost of putting the system out of service for the time needed to make such repairs. This presents a need for an interim repair at minimum expense to the utility until the sleeve can be replaced at a more appropriate time such as during routine scheduled maintenance. Since pressurizer components in nuclear power plants become radioactive, after they have been in operation, it is preferable that any repairs can be made without the need for personnel to enter the pressurizer so as to minimize exposure. Repair apparatus and methods which applicants are aware of include the following.
U.S. Pat. No. 4,255,840 discloses a method of replacing an immersion heater in a pressurizer for a nuclear power plant.
U.S. Pat. No. 4,615,477 discloses a method for replacing tubes in a header or drum without entirely removing the old tube.
U.S. Pat. No. 4,440,339 discloses a method for repairing the housing of a control rod driving system of a nuclear reactor.
U.S. Pat. No. 4,203,185 discloses a method for sealing tube plate apertures in heat exchangers. This is directed to the formation of an insulating space over the major part of the thickness of the tube plate rather than to welding techniques.
U.S. Pat. No. 3,934,787 discloses a method for welding steel pipe to a pressure vessel. An opening is cut in the wall of the vessel and a connection member of lenticular shape having a thickened central portion is joined to the pipe and vessel.
U.S. Pat. No. 4,178,966 discloses a tube plug having a lip that limits the distance the plug can be inserted into the tube and supplies filler material at the weld zone between the plug and tube.
U S. Pat. No. 4,811,759 discloses a device for closing holes in a tubular plate of a steam generator. A sleeve receives a plug which is swaged and welded in position in the sleeve. The sleeve is fastened in the tube hole by expanding that portion of the sleeve in the tube plate.
Pending U.S. application assigned Ser. No. 07/468,448 and filed Jan. 22, 1990, now U.S. Pat. No. 5,094,801, on which one of the present applicants is a coinventor, discloses a replacement heater sleeve for a nuclear reactor coolant system pressurizer and method for replacing a damaged original heater nozzle.
The known art is directed mainly to the replacement of sleeve penetrations or the cutting of a new opening and welding of a pipe to the opening to create a new penetration. This leaves a need for a method and apparatus to close a penetration at minimum cost to the utility until a more appropriate time for permanent repair and replacement of the sleeve.